GK 700RB bad +15 volt supply

[booj]

700RB Power Amp and supply.pdf700RB Preamp.pdf



The +15v. supply is about .6v. with the preamp board connected, and goes up to about 8 volts with the preamp board dis-connected. D5, D6 and Q19 pass diode checks. D12 checks ok and is new.
>Collector Q9 - 37.7
>Cathode D12 - 9.35
>Cathode D5 / Anode D6 - 8.66
>Cathode D6 - 7.97
>Emitter Q9 – 8.98
Possibly U1 is bad? Pin 2 reads exactly the same as pin 8. 7.98 v.

[Enzo]

It should be +15, but even without the load it can only make 8 volts.

1N965 D12 sets the voltage, and it appears to be sitting at only 9v, so tough to get 15 from that.


Diode tests are great for finding whacko parts, but they wonlt tell you a thiing about the part under load. PArts that check OK on junction drop can still fail miserably as soon as you try to drag some current through them. A zener may check like a diode OK, but that won;t tell you that it now wants to zene at 6v instead of 16v. Or whatever.

You have two identical circuits. COmpare the working -15v supply as a reference.

D12 may be faulty, but then it may not be getting the voltage it needs int he first place, so check R32. Probably not, but C12 could be leaking like a screen door, lift one end and see.


Q19 may check OK, but it may not be able to conduct enough.

ANd don;t forget little R33, 2.7 ohms. If that opens or shoots up in value, then the supply will TRY to put 15v out through R32 and D5 D6. And it won;t find that very easy.

[booj]

D12 may be faulty, but then it may not be getting the voltage it needs int he first place, so check R32. Probably not, but C12 could be leaking like a screen door, lift one end and see.


Q19 may check OK, but it may not be able to conduct enough.

ANd don;t forget little R33, 2.7 ohms. If that opens or shoots up in value, then the supply will TRY to put 15v out through R32 and D5 D6. And it won;t find that very easy.

I replaced D12 again with a 1 watt 16 volt zener and got 9.3 volts from the supply.
R32 seems ok. Measures 4.7k.
C12 could be leaky. I'll replace it, although when power is turned off the voltage across it doesn't go to zero very fast, but drains down slowly.
Q19 does check ok with a diode check. I haven't had much experience with leaky transistors in a circuit like this, but it seems a likely prospect. It will get replaced.
R33 measures 2.7 ohms
At this point I have to order parts. When I install them I'll post the results if I can find this thread.

[Enzo]

2SC4793 seems liie overkill here. 230v part in a 38v circuit. 1 amp rating.

Nothing wrong with ordering the exact part, but to see if the thi8ng would work, I bet you have something in your shop that would work. AFter all, this is just a voltage regulator circuit. DOn;t need a lot of bandwidth for DC. Even if you want to get the real part, a temporary sub just to see if it is what you need might be prudent.

TIP29C, TIP31C, TIP41C are all 100v parts, which should be plenty. If you want voltage, the MJE15032 is 250v rated, and at tons of current.

And there are any number of TO220 NPNs used as drivers in power amps that ought to work. 2SC3298 comes to mind from my drawers.